This invention relates to high temperature superconductors (HTS), and more particularly to superconducting layers and methods of making superconducting layers.
Coated conductors, comprising a single or multiple combinations of a biaxially textured high temperature superconductor (xe2x80x9cHTSxe2x80x9d) layer on a thin buffer layer and a substrate tape, are a cost-performance-effective technology for manufacturing long length flexible HTS wire for magnet, coil and power applications. For example, these conductors should be useful for power transmission cables, rotor coils of motors and generators, and windings of transformers, as well as for magnets for medical magnetic resonance imaging (MRI), magnetic separation, ion-beam steering and magnetic levitation. Particularly of interest here are applications that use alternating current (ac) currents and fields, or fast ramps of current and field, for example ac power transmission cables, transformers, faultcurrent limiters, magnetic separation magnets and high-energy physics magnets.
Some background on biaxially textured high temperature superconducting xe2x80x9ccoated conductorsxe2x80x9d is known. Such coated conductors include at least, for example, a substrate and a superconducting layer (such as YBCO; YBa2Cu3Ox, or Yttrium-Barium-Copper-Oxide) deposited thereon. One or more buffer layers may be included between the substrate and the superconductor material. An advantage of such materials as YBCO films is the very high critical current densities attainable, particularly in magnetic fields. Other related superconducting materials that can be used are (RE)BCO (REBa2Cu3Ox, where the Y has been partially or completely replaced by rare earth elements, RE). As information as to the requirements for commercial application, and limitations on conductor technology has become available, the potential for low production costs of these rare-earth superconducting materials (including YBCO) has also become of interest in further development.
Certain challenges in this field include the need for cost effective methods for producing chemically compatible biaxially textured buffer layers, as well as the need to deposit sufficient thickness of the high critical current density superconducting layer. Regarding the first objective, it appears that deformation textured substrates with epitaxial buffer layers can be made cost effective. In addition, ion beam assisted deposition of thin MgO layers with epitaxial top layers may prove to be economically viable.
Regarding the need to deposit thick layers of superconductor precursor compositions, a number of techniques have been evaluated. Chemical vapor deposition (CVD) is not considered a competitive method at this time, due to the very high cost of precursor materials. Most physical vapor deposition (PVD) methods, (for example, pulsed laser ablation, reactive sputtering and electron beam evaporation) are limited by deposition rate, compositional control, and high capital costs. A possible economical PVD method would be thermal or electron beam evaporation of the rare earth elements, copper and barium fluoride, known as the xe2x80x9cbarium fluoridexe2x80x9d process. This process appears to be more rapid than direct PVD methods, but capital costs and control system costs are still likely to be too high. Additionally, the deposited precursor composition must subsequently be reacted in a separate furnace system to form the HTS film.
Solution deposition methods have been evaluated, and these appear to offer much lower costs, since vacuum systems are eliminated. Thus, capital costs are not as high, and deposition rates not as low, as other methods using vacuum systems. Trifluoroacetate (TFA) solution processes offer low costs for precursor compositions, high deposition rate, and non-vacuum processing advantages.
For commercial processes, it is desirable to have a process, which can be used to produce high quality oxide layers at a high rate, to produce a desired film thickness, and a film with high critical current density for superconducting applications.
The invention provides a process for fabricating dense, highly textured metal oxide films from precursor films containing metals or metal oxides and metal halides. The final thicknesses of such films are preferably between about 1 micron and about 5 microns. The specific superconductors of interest are high temperature superconductors of the class of rare-earth barium cuprate species (REBCO), including, for example, YBa2Cu3O7xe2x88x92x (YBCO), and other known superconducting materials, including versions doped with other species. Of particular interest are those materials having superconducting transition temperatures, Tc, above about 77 K. The most useful buffered substrates for such films are biaxially textured, providing an epitaxial growth template for achieving maximum attainable critical current densities (Jc).
The process can be used for the formation of superconducting films on both single crystal substrates (such as SrTiO3, LiAlO3, and the like), and composite substrates comprising a metal or metal alloy with a chemically and structurally compatible buffer layer (for example, Ni/CeO2/YSZ/CeO2, and similar structures). The process is applicable to batch processing of individual samples or continuous processing of continuous lengths of material (that is, continuous lengths of wire or tape). A key aspect is the ability to grow thick superconducting films with the same dense microstructure, crystalline texture and current density as previously obtained in thin films, that is, films of thickness less than 0.4 micrometers.
In one aspect, the invention provides a method for forming an oxide layer. The method includes disposing an oxide layer precursor on a surface of a first layer (e.g., a substrate, such as a biaxially textured metal or metal alloy, such as a face centered cubic metal or metal alloy, such as a nickel-containing alloy) at ambient temperature; increasing the ambient temperature to a process temperature; and treating the precursor for a time and under formation conditions sufficient to form the oxide layer, where the formation of the oxide layer is substantially inhibited until the ambient temperature reaches the process temperature. The inhibition of oxide layer formation can be by modulating the rate of introducing a reactant to the first layer, and modulating the rate of removing a product from the first layer. The rate of reactant introduction and product removal can be by modulating the flow rate of a process gas flowing proximate to the surface. For example, the flow rate of the process gas can be lower before the ambient temperature reaches the process temperature (for example, from about 600 to about 850xc2x0 C., or from about 650 to about 850xc2x0 C., or from about 700 to about 800xc2x0 C.) than it is after the ambient temperature reaches the process temperature. The precursor can react with a gaseous reactant to form the oxide layer, and the formation of an oxide layer can proceed with the generation of a gaseous product. The precursor can include soluble compounds of a first metal (for example, a copper compound, such as copper oxide), a second metal (for example, an alkaline earth metal, such as barium fluoride) and a rare earth metal. The oxide layer can have a superconducting transition temperature of more than about 77K, and the oxide layer can form a film having a thickness of at least about 0.6 microns, and the oxide layer can have a critical current density of at least about 5xc3x97105 A/cm2. The first layer can further include a buffer layer disposed on the substrate, which can include at least one layer of a metal oxide (such as cerium oxide, or a further layer of yttria-stabilized zirconia). The process gas can include water vapor (which can have a partial pressure of from about 5 to about 150 Torr, or from about 5 to about 100 Torr, or from about 10 to about 50 Torr, or from about 15 to about 20 Torr at ambient temperature) and oxygen (which can have a partial pressure of from about 5 mTorr to about 8000 mTorr, or from about 5 to about 800 mTorr, or from about 5 to about 200 mTorr, or from about 5 to about 100 mTorr, or from about 50 to about 100 mTorr in 1 atmosphere total process gas pressure).
In a further aspect, the invention provides a coated conductor article made by the methods disclosed herein.
As used herein, xe2x80x9cbiaxially texturedxe2x80x9d refers to a surface for which the crystal grains are in close alignment with a direction in the plane of the surface. One type of biaxially textured surface is a cube-textured surface, in which the crystal grains are also in close alignment with a direction perpendicular to the surface. Examples of cube-textured surfaces include (100)[001] and (100)[011] surfaces, and an example of a biaxially textured surface is the (113)[211] surface. As used herein, xe2x80x9cepitaxial layerxe2x80x9d refers to a layer of material, the crystallographic orientation of which is directly related to the crystallographic orientation of the surface of a layer of material onto which the epitaxial layer is deposited. For example, for a multi-layer superconductor having an epitaxial layer of superconductor material deposited onto a substrate, the crystallographic orientation of the layer of superconductor material is directly related to the crystallographic orientation of the substrate. Thus, in addition to the above-discussed properties of a substrate, it can also be desirable for a substrate to have a biaxially textured surface or a cube textured surface. Texturing in HTS films has been demonstrated by epitaxial growth of the superconductor on appropriate templates. Early work used single crystal oxide substrates as the HTS growth template. For many practical applications, the substrate must be flexible. A well-oriented template can be achieved by means of ion beam assisted deposition (IBAD) of oxide buffer layers on random polycrystalline metal substrates. High quality epitaxial superconducting films have been grown on such tapes. In alternative and preferred embodiments, texturing in substrates can be induced using the deformation texturing (xe2x80x9cDeTexxe2x80x9d) process (as set forth hereinbelow) and epitaxial deposition of buffer layer(s) and subsequent functional layer(s) can be accomplished as also described hereinbelow.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.